The present invention relates to a liquid crystal display device (LCD) permitting reflection-mode display and a method for fabricating such a liquid crystal display device.
In recent years, LCDs have been broadly used for a variety of apparatuses including word processors, personal computers, TV sets, video cameras, still cameras, monitors for cars, portable office automation (OA) appliances, and portable game machines.
The LCDs do not emit light themselves, unlike cathode ray tubes (CRTs) and electro luminescence (EL) devices. Therefore, in the case of a transmission LCD using pixel electrodes (transparent electrodes) made of a transparent conductive material such as indium tin oxide (ITO), an illuminator such as a fluorescent tube (so-called backlight) is disposed at the rear of a liquid crystal panel so as to effect display using light emitted from the illuminator. Such a transmission LCD exhibits higher display quality but dis-advantageously consumes larger power, compared with a reflection LCD described hereinafter. A backlight normally consumes 50% or more of the total power of the LCD.
In order to solve the above problem of the transmission LCD, there have recently been developed a reflection LCD including pixel electrodes (reflection electrodes) made of a material having a reflection characteristic such as a metal and a transmission/reflection combination type LCD including both a transparent electrode and a reflection electrode for each pixel.
The LCD permitting reflection-mode display as described above includes a reflector for reflecting ambient light. Such a reflector may be placed inside a pair of substrates constituting a liquid crystal panel (internal type) or outside the rear substrate (on the side of the substrate opposite to the side of a liquid crystal layer) (external type). The internal type is advantageous in being free from an occurrence of double image due to the thickness of the substrate (typically, glass substrate). In addition, since the internal type reflector is typically made of a metal exhibiting electrical conductivity such as aluminum, it can also be utilized as pixel electrodes (or part of pixel electrodes). This simplifies the construction.
In order to realize display with a good paper-white property in the reflection mode, the reflector should preferably have an appropriate diffuse reflection characteristic (light distribution). If the reflection plane is close to a mirror plane, it mostly returns specular reflection (mirror reflection), causing a trouble of reflecting ambient images in some cases. In reverse, if the diffuse reflection characteristic is too large, the brightness lowers. It is therefore preferable to adjust the diffuse reflection characteristic so that good paper white property and brightness can be obtained.
A method for forming an internal type reflector (or reflection electrodes) is disclosed in Japanese Laid-Open Patent Publication No. 9-292504 (corresponding U.S. Pat. No. 5,936,688) of which applicant is the same as the assignee of the present application. U.S. Pat. No. 5,936,688 is incorporated herein by reference. In the disclosed method, a photolithography process and a heat treatment process are combined as described below.
A photosensitive resin film formed on a substrate is exposed to light via a photomask having a predetermined pattern and developed, to form a convex/concave profile corresponding to the predetermined pattern. The photomask, for example, has circular light-shading spots randomly distributed therein if a positive photosensitive resin is used. The photosensitive resin film having the convex/concave profile is then heat-treated to smooth the convex/concave profile utilizing the thermal deformation of the resin. A metal film is then formed over the resultant smooth convex/concave (continuously waved) surface, and patterned to a predetermined shape corresponding to the shape of pixels thereby to form a reflector.
In the exposure of a photosensitive resin, an exposure system (i.e., aligner) such as a stepper or a large-scale one-shot (full plate) exposure system is normally used. A stepper is preferably used for forming a photosensitive resin film as described above, that is, the photosensitive resin film having the convex/concave surface that determines the surface profile of the reflector required to have an appropriate diffuse reflection characteristic. The reason is as follows. A large-scale one-shot exposure system allows a large area to be exposed to light at one time, but has large in-plane variations in light intensity and degree of collimation. This makes it difficult to obtain a reflector having a good diffuse reflection characteristic. The convex/concave profile of the surface of the photosensitive resin film substantially determines the surface profile of the reflector. Accordingly, if the convex/concave profile lacks uniformity over the entire surface, the diffuse reflection characteristic of the reflector varies, resulting in failure in uniform display. If a large-scale one-shot exposure system is employed, the resultant reflection characteristic will be such that only the center of an exposed area is bright while the periphery thereof are dark, for example. It is therefore difficult to obtain a reflector suitable for practical use.
In other words, precise profile control is required in order to form an underlying layer having a predetermined surface profile for controlling the surface profile of the reflector so that the reflector has an appropriate diffuse reflection characteristic. This is different from the case of forming contact holes that do not directly influence the display. If the exposure is made using light having large in-plane variations in intensity and degree of collimation, it is impossible to process the surface of the underlying layer into a predetermined profile.
In a stepper, light from a light source is nearly collimated via a lens system, whereby in-plane variations in light intensity and degree of collimation are made small. The stepper however has a disadvantage of exposing only a small area at one time. For example, as shown in FIG. 23A, a region 87 a stepper can expose at one time is only about 6 inches (about 152.4 mm) in diameter. This means that a region 86 allowed for formation of a convex/concave profile is a square of about 6 inches in diagonal at maximum. In order to expose an area larger than 6 inches in diameter using the stepper, division exposure as shown in FIG. 23B is required, where an area is divided into sub-regions for individual exposure. More specifically, a first sub-region 88a is exposed to light in the first exposure step (an exposure range 89a; an exposure center 90a), and thereafter a second sub-region 88b is exposed to light in the second exposure step (an exposure range 89b; an exposure center 90b).
However, the exposure using a stepper still exhibits an in-plane variation of light (image distortion; while the degree of collimation of a light ray is high in the center, it is low in the periphery). The resultant light pattern obtained using the stepper is distorted from an ideal pattern that would be obtained if a photomask is illuminated with completely collimated light (the pattern of light-transmitting spots of the photomask). The distortion is greater in a portion closer to the periphery. For example, if a mask for illuminating a plurality of circular spots is used, circles are obtained in the center of an exposed area but ellipses instead of circles are formed in the periphery. In the case of one-shot exposure, when a reflector is formed on the resultant convex/concave surface, the reflection characteristic changes as the convex/concave pattern changes from circles to ellipses from the center toward the periphery of the exposed area. However, this change in reflection characteristic is continuous and thus hardly recognized as a change in display quality.
In the case of division exposure, however, the following problem arises. When a reflector is formed on the convex/concave surface obtained by the division exposure, the reflection characteristic of the reflector changes discontinuously, and thus the seam of division exposure is observed as a change in display quality. This is because, at each seam of division exposure, an ellipse having one major axis direction (direction of distortion of the convex/concave pattern) is adjacent to an ellipse having another major axis direction. That is, when a reflector is formed on a photosensitive resin film subjected to division exposure, a seam (boundary) 91 shown in FIG. 23B is revealed between the first and second exposure steps.
The distribution of the degree of collimation of light generated by a stepper can be made uniform to some extent by correcting lens distortion. However, the convex/concave profile changes with a minute change in light intensity and degree of collimation, and the reflection characteristic largely varies with a small change in convex/concave profile. It is therefore difficult to form a reflector having a practical reflection characteristic employing a division exposure method even by correcting lens distortion.
The in-plane variations in light intensity and degree of collimation generated by use of the stepper can be reduced by another way. That is, the regions 88a and 88b exposed in one exposure step (by one shot) may be made smaller. However, this increases the number of exposure steps and accompanying alignment steps, resulting in markedly lowering the production efficiency. In addition, improvement of the degree of collimation is limited in this method.
Another problem of division exposure is as follows. The sub-regions exposed during the respective exposure steps somewhat overlap with each other in consideration of possible displacement of exposure regions (photomask alignment error).
Hereinafter, each overlap portion between the exposed regions by division exposure is referred to as a seam portion (or a boundary portion).
For example, assume that photomasks 82a and 82b shown in FIG. 24 are used to perform division exposure and a seam portion 1 is formed. The portion of a photosensitive resin 10 film constituting the seam portion 1 is twice exposed to light transmitted through light-transmitting spots of the photomasks 82a and 82b in the two exposure steps. As a result, after development, the surface profile of the seam portion 1 of the photosensitive resin film is completely different from that of the other portion of the photosensitive resin film. The reflector to be formed on the thus-formed photosensitive resin film will have a reflection characteristic that varies between the portions located above the seam portion 1 and the other portions. In particular, if a seam portion 91 is formed in a pixel region as shown in FIG. 25, the change in reflection characteristic at the seam portion 91 is more likely to be observed, thereby markedly lowering the display quality.
A method for making the seam portion less visible is disclosed in Japanese Laid-Open Patent Publication No. 11-7032. In this disclosure, a photosensitive resin is subjected to division exposure so that at least one pixel column that constitutes a seam portion includes pixels subjected to different exposure steps (that is, the seam portion is formed to be zigzagged every pixel). This publication also discloses a method of division exposure where each pixel of at least one pixel column constituting a seam portion is subjected to different exposure steps (that is, the seam portion is formed inside the pixel). However, in the zigzagging method, high-precision alignment of a photomask is required both in the row and column directions. This makes it difficult to obtain satisfactory productivity. In the method of forming the seam portion inside a pixel, the reflection characteristic at the seam portion largely changes with a minute deviation in alignment of the photomask. High-precision alignment is therefore required to make the seam portion less visible. This also makes it difficult to obtain satisfactory productivity.
An object of the present invention is providing a liquid crystal display device permitting reflection-mode display that has good productivity and minimizes lowering in display quality at seam portions generated by division exposure, and a method for fabricating such a liquid crystal display device.
The method for fabricating a liquid crystal display device including a plurality of pixels arranged in a matrix constituting a plurality of pixel columns, each of the plurality of pixels having a reflection region permitting reflection-mode display, the reflection region including an insulating layer having a convex/concave surface and a reflector formed on the convex/concave surface of the insulating layer includes the step of forming the insulating layer having the convex/concave surface. The step of forming the insulating layer includes the steps of: forming a photosensitive resin film; exposing a first region of the photosensitive region film via a first photomask (first exposure step); exposing a second region of the photosensitive region film that includes a region different from the first region via a second photomask (second exposure step); and developing the exposed photosensitive resin film, wherein the first and second exposure steps are performed so that a boundary portion defined as an overlap between the first region and the second region or a space between the first region and the second region is located to overlap with at least part of an inter-column space between adjacent pixel columns among the plurality of pixel columns.
The first and second exposure steps may be performed so that the boundary portion overlaps with the inter-column space and part of the reflection regions of the pixels on both sides of the inter-column space. Alternatively, the first and second exposure steps may be performed so that the boundary portion is located to be within the range of the inter-column space.
In the first and second exposure steps, a pattern via which the boundary portion of the photosensitive resin film is exposed to light is preferably be the same as a pattern via which portions of the photosensitive resin film corresponding to the inter-column spaces other than the inter-column space corresponding to the boundary portion.
In the first and second exposure steps, the portions of the photosensitive resin film corresponding to the inter-column spaces may be exposed to light having a substantially uniform intensity distribution. Alternatively, in the first and second exposure steps, the boundary portion of the photo-sensitive resin film and the portions of the photosensitive resin film corresponding to the inter-column spaces other than the inter-column space corresponding to the boundary portion may substantially not be exposed to light.
The portions of the photosensitive resin film corresponding to all the inter-column spaces formed by the plurality of pixels may be removed.
Preferably, the first and second exposure steps are performed so that the boundary portion of the photosensitive resin film is not double exposed to light.
The liquid crystal display device of the present invention includes a plurality of pixels arranged in a matrix constituting a plurality of pixel columns, each of the plurality of pixels having a reflection region permitting reflection-mode display, the reflection region including an insulating layer having a convex/concave surface and a reflector formed on the convex/concave surface of the insulating layer, wherein the insulating layer includes: a first region allowing the overlying reflector to exhibit a first reflection characteristic; a second region allowing the overlying reflector to exhibit a second reflection characteristic; and a third region formed between the first region and the second region, and the third region is located to overlap with at least part of an inter-column space between adjacent pixel columns among the plurality of pixel columns.
The third region may overlap with the inter-column space and part of the reflection regions of the pixels on both sides of the inter-column space. Alternatively, the third region may be located to be within the range of the inter-column space.
Preferably, a portion of the insulating layer corresponding to the at least part of the inter-column space overlapping with the third region has the same convex/concave profile as a portion of the insulating layer corresponding to the inter-column space that does not overlap with the third region.
The third region of the insulating layer may have a substantially flat surface.
At least part of the insulating layer corresponding to all the inter-column spaces formed by the plurality of pixels may have been removed.
Preferably, the device further includes: switching elements provided for the respective plurality of pixels; scanning lines for applying a scanning signal to the switching elements; and signal lines provided to intersect with the scanning lines for applying a display signal to the switching elements, wherein the scanning lines and the signal lines run between the plurality of pixels.